A transmission line is widely used to guide a radio frequency signal ("RF") between two points along a particular path in an RF device or RF system. Various configurations of transmission lines have been developed for different RF applications, including but not limited to, open two-wire line, rectangular waveguide, ridged waveguide, coaxial cable, single-dielectric microstrip line, and triplate stripline. For a given application, the use of a particular type of transmission line is usually based on device parameters such as the operating frequency, power level, mass, volume, and performance requirements.
In particular, multilayer transmission lines have been used to reduce the size and mass of microwave circuits and to improve their performance. Commonly used transmission line configurations include a microstrip transmission line which is constructed by forming a conducting circuit on top of a dielectric substrate with a ground layer on the bottom of the substrate and a stripline transmission line which has a flat conductor disposed between two ground planes. Impedance matching may be accomplished by changing the dimension of the conducting features or strips. Such multilayer transmission lines may be constructed on metal-clad printed-circuit boards, or metal-clad planar dielectric materials and can be implemented in applications where volume and cost constraints predominate such as miniature microwave integrated circuits ("MMIC").
Many multilayer circuit configurations use stripline transmission lines in which the ground planes separate the various signal layers. Interlayer connections between the stripline transmission lines on different layers are conventionally accomplished by using vertical conductors directly connecting the center conductors of the stripline transmission lines of interest. These vertical connections, sometimes referred to as "vias", may be fabricated by using a "plated-through-hole" technique. The presence of vertical conductors in multilayer configurations adds manufacturing complexity and can lead to degraded device performance at high frequencies. In addition, as the frequency increases, additional vertical conductors may be required for suppressing unwanted modes. This further complicates the device and increases manufacturing cost.
Alternatively, the interlayer connections can be implemented by electromagnetic coupling without direct physical contact between stripline transmission lines on different layers. A stripline transmission line supports electromagnetic wave propagation between surfaces of both a conductor and a respective ground plane. An opening or slot formed in the ground plane can disturb the electromagnetic field to cause electromagnetic radiation at the slot. This radiation couples the electromagnetic energy from the stripline transmission line. If a second stripline transmission line is placed on the opposite side of the ground plane, the radiated energy from the slot will be coupled to the second transmission line.
Franchi et al. discloses a multilayer interconnecting scheme based on the above electromagnetic coupling through a rectangular slot of length of a quarter wavelength in a common ground plane in U.S. Pat. No. 5,093,636. Two striplines on two adjacent layers each are terminated by a longitudinal coupling unit which consists of an impedance matching section directly connected to the stripline and a coupling section extended from the impedance matching section. Both the impedance matching section and the coupling section are of a quarter-wavelength in length. The coupling sections of the two striplines are aligned with the rectangular slot in the common ground plane between two striplines.
U.S. Pat. No. 5,471,181 to Park describes a multilayer microwave circuit having a U-shaped coupling slot with an effective electrical length of one half wavelength in a common ground plane to interconnect two adjacent layers. A cavity formed by vertical conductors is implemented to enclose the U-shaped slot to reduce undesirable coupling to other transmission line modes.
The present invention uses a novel impedance matching and transition geometry to implement the transition between two stripline transmission lines on two adjacent signal layers in a multilayer circuit based on the electromagnetic coupling.